JP1992-312221A discloses a shock absorber interposed between a vehicle body and a wheel to damp road vibration input to the wheel. This shock absorber is applied to a front fork for suspending a front wheel of a two-wheeled vehicle and includes a shock absorber main body composed of a vehicle-body side tube and a wheel side tube to be slidably inserted into the vehicle-body side tube via a pair of upper and lower bearings as shown in FIG. 1 of the above document.
The upper bearing is mounted on the outer periphery of the wheel side tube and the lower bearing is mounted on the inner periphery of the vehicle-body side tube, and a working fluid stored in a lubrication clearance formed between these bearings functions as a lubricant to provide good slidability.
An upright damper including a cylinder which stands in an axial center part of the wheel side tube and in which the working fluid is filled, a piston which is held in sliding contact with the inner periphery of the cylinder and partitions the interior of the cylinder into two working chambers, and a rod which intrudes into and exits from the cylinder via the piston is housed in the shock absorber main body. A reservoir chamber is formed between the shock absorber main body and the damper.
The reservoir chamber is composed of an air chamber and a working fluid chamber, and the working fluid that becomes excessive or insufficient in the cylinder by the amount of displacement of the rod is compensated via a base member provided at a bottom part of the cylinder, thereby carrying out a volume compensation for the damper.
The working fluid chamber of the reservoir chamber communicates with the lubrication clearance via a flow path, and a check valve for permitting only a movement of the working fluid from the working fluid chamber to the lubrication clearance is provided at an intermediate position of the flow path.
When the front fork is compressed, the working fluid that becomes excessive by the intruding amount of the rod flows out into the working fluid chamber via the base member to increase the volume of the lubrication clearance. This causes a pressure difference between the working fluid chamber and the lubrication clearance, whereby the check valve is opened and the working fluid flows into the lubrication clearance via the flow path.
On the other hand, when the front fork is extended, the lubrication clearance is compressed and the check valve is biased in a closing direction by an inner pressure of the lubrication clearance, wherefore communication of the flow path is blocked.
Accordingly, by the front fork being repeatedly extended and compressed, a so-called pumping action is performed, whereby the working fluid is pumped up into the lubrication clearance, a fluid level in the lubrication clearance becomes higher and the working fluid can be more easily supplied to a sliding surface of the upper bearing.
However, if the pressure difference between the working fluid chamber and the lubrication clearance is small, the check valve is not opened, wherefore the working fluid is not sufficiently supplied to the sliding surface of the upper bearing and lubrication may possibly become insufficient.
Accordingly, the following structure is disclosed in FIG. 3 of the above document. The working fluid chamber is partitioned into upper and lower sections by a rod guide, and a communication path allowing communication between the upper and lower working fluid chambers is formed in the rod guide. Further, a damping valve for giving resistance when the working fluid moves from a lower working chamber to an extension side working chamber along the communication path is provided, and the lower working fluid chamber communicates with the lubrication clearance via the check valve.
In this way, when the front fork is compressed, an inner pressure of the lower working fluid chamber is increased to cause a pressure difference, thereby enabling the working fluid to be more reliably supplied to the sliding surface of the upper bearing.